Method for producing patina on a zinc surface and article so formed

ABSTRACT

Method for forming a smooth, even, corrosion resistant patina on a zinc surface by abrading or otherwise mechanically graining the surface, and thereafter forming an even zinc phosphate coating thereon, particularly, a zinc phosphate coating in the presence of a nickel ion, which coating is preferably substantially identical in appearance to the natural patina formed by exposure to the atmosphere, and article so formed.

nited States Patent [1 1 Dollar et a1.

[4 1 Oct. 23, 1973 METHOD FOR PRODUCING PATINA ON A ZINC SURFACE AND ARTICLE SO FORMED I [75] Inventors: David L. Dollar, Muncie, Ind.;

Garland D. Richardson, Lakewood, C010.

[73] Assignee: Ball Corporation, Muncie, Ind.

[22] Filed: Sept. 14, 1971 21 Appl. No.: 180,527

[52] US. Cl. 148/6.l5 Z, 148/315 [51] Int. Cl. C231 7/12 [58] Field of Search l48/6.15 R, 6.15 Z,

[56] References Cited UNITED STATES PATENTS 3,592,701 7/1971 Pekar l48/61.15 R

3,413,231 11/1968 Kolodny 148/65 R X 3,240,633 3/1966 Gowman 148/615 Z 2,121,574 6/1938 Romig 148/6.15 Z

OTHER PUBLICATIONS Wiederholt, Chemical Surface Treatment of Metals, Robert Draper Ltd, 1965 p. 97

Primary ExaminerRalph S. Kendall Att0rneyGilbert E. Alberding 10 Claims, No Drawings METHOD FOR PRODUCING PATINA ON A ZINC SURFACE AND ARTICLE SO FORMED BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to a method for forming an artificial patina on zinc surfaces and, more particularly, relates to a method and apparatus for treating zinc surfaces wherein the surfaces are first mechanically grained and thereafter treated with a zinc phosphate composition, including a nickel ion, to provide a surface which is substantially identical in appearance to a weathered zinc surface, and'which is resistant to corrosion, and to articles so formed. i

2. Discussion of the Prior Art It is well-known that zinc and zinc alloys, on structural materials, are highly desirable in that the surface of the zinc reacts with the atmosphere to form a carbonate surface layer which is resistant to corrosion and produces a pleasing relatively dark gray patina. Not only is the coloration of zinc considered aesthetically pleasing, but the surface heals itself when damaged by a mechanism identical to that which produces the orginal patination. Further, the surface layer effectively seals the underlying zinc from corrosive attack. Ac-

cordingly, the life span of architectural zinc, i.e., zinc roofs, is measured in centuries.

Despite the advantages accruing to zinc as an architectural material, there are certain very real, but shortlived, disadvantages. Primarily, the zinc material displays an undesirable bright metallic appearance when first installed. This brightness persists for about six months while the surface patina is formed. While the initial brightness is quite short-lived relative to the useful life of the zinc after the patina is acquired, and even though the brightness is objectionable only from an aesthetic viewpoint and in no way comprises the function of the material, the fact remains that the brightness exists during'a particularly sensitive period. Builders, architects and others are hesitant to recommend zinc materials with the knowledge that an owner of a new structure must wait six months or longer for the zinc to form the patina and take on the final, desired appearance.

In addition to the less desirable appearance of new, bright metallic zinc, it is also true that the zinc is more susceptible to undesirable reaction with, for instance, fuel oil fumes, galvanically active other metals or atmospheric pollution while in the bright, unweathered state. The weatheredsurface, being essentially chemically passive, is much less susceptible to undesirable reaction with other contaminating materials which may cause undesirable discoloration.

Numerous techniques have previously been used to treat zinc surfaces. Phosphating has been utilized as a primer for painting or otherfurther treatment, but has not been attributed with producing any particulariincrease in corrosive resistance, and is entirely unknown as an agent capable of providing a suitable surface appearance in itself. As ordinarily applied, phosphate coatings produce a rather mottled, uneven coating. On the other hand, chromatized coatings have been employed in an attempt to increase both resistance to corrosion and to provide a pleasing coloration on the surface of zinc. While the chromatized coatings have been somewhat successful on the first object, they have not been particularly successful on the latter object, and

certainly not with regard to duplicating the natural patina occurring on zinc when weathered. Typical of the prior art dealing with this concept are U.S. Pat. Nos. 3,333,988, 3,338,755 and 3,413,231. Also germane and particularly in point in current is the publication New Zinc Roofing Techniques", January-June, I970, Vieille-Montagne.

Summarily, it is recognized that treatment of zinc to initially duplicate the appearance and corrosion resistance which occurs over a period of time when exposed to the weather is a highly desirable end. However, other than employing phosphates as priming material or chromates to imitate without duplicating natural weathering, the prior art appears to be substantially devoid of success in obtaining the desired results.

SUMMARY OF THE INVENTION This invention provides an improved treatment for producing a surface reactive coating on metals comprising zinc. The surface treatment is even, aesthetically pleasing and, preferably, may duplicate the coloration of the natural weathering of zince metals if desired. Further, the surface coloration, which is obtained by a phosphating of the zinc surface after a critical mechanical graining step, provides a surface chemical passivity comparable to that obtained by natural weathering of zinc. In the preferred embodiment wherein the surface treatment is accomplished utilizing a nickel ion, the texture andcoloration of the surface is essentially identical to that of natural weathering.

It is therefore an object of this invention to provide an improved method for treating zinc metals.

It is another object of this invention to provide an improved pre-treated zinc material which is essentially identical in appearance to the naturally weathered zinc though of a different chemical composition.

It is still. another object of this invention to provide an improved method for preparing the zinc surface by mechanical graining in order that subsequent phosphating provides an even, smooth protective surface substantially identical in appearance to that obtained by natural weathering.

Yet another object of this invention is to provide a means for phosphating zinc surfaces to obtain an even, pleasing texture and coloration on such surface.

With these and other objects in view, it will become apparent to those skilled in the art, from the description herein, that the invention resides in the novel combination and sequence of steps as hereinafter described and more particularly defined by the appended claims. It is understood that the precise, preferred embodiments of the invention herein disclosed are subject to changes and alterations, as will be obvious to those skilled in the art, while within the scope of the inven tion.

PREFERRED EMBODIMENT OF THE INVENTION According to the present invention, a clean zinc or zinc alloy material in sheet, plate, strip or other form is mechanically grained by an oscillating brush in the presence of a pumice slurry. This graining step is critical to the invention in that the texture and appearance of the resulting article is highly dependent on suface grain and cleanliness. After cleaning to remove foreign material introduced during the graining operation, the zinc metal is then exposed to a zinc phosphate solution, rinsed and dried. As a result of the unique-for purposes of later phosphating-physical condition of the zinc surface after mechanical graining, the zinc phosphate solution provides an unusually even coating which is essentially identical, except in coloration, to the natural weathering of zinc. By using a particularly preferred zinc phosphate solution incorporating a nickel ion, even the coloration of the coating can be essentially identically matched to that of naturally weathered zinc. It is, or course, recognized that natural weathering of zine produces a coloration which varies over a somewhat limited range depending upon the atmospheric conditions, the exact zinc alloy and other such variables. However, those skilled in the art will recognize that the medium dark gray patina produced by other chemical treatments heretofore employed differ from natural weathering while those of the instant invention can be made identical in appearance to naturally weathered zinc. In any event, without the novel mechanical graining step, the texture of the phosphated zinc surface has not been at all similar to that which is produced by natural weathering.

The mechanical graining may be accomplished by many known methods, though it is preferred that a thorough abrasive brushing be utilized preferably in the presence of a pumice slurry. Such slurries may be produced from many materials. A typical, illustrative slurry would contain fifty pounds of Italian pumice (FFF), pounds of 7/10 quarts and three pounds of trisodium phosphate.

While the mechanical graining of the surfaces will provide a distinct fine, dense phosphating of the zinc surface with any of the known phosphating compositions in the usual manner, i.e., at a pH of 3 to 4, etc., and, accordingly, will provide increased corrosion resistances as opposed to the simple primer function heretofore attributed to phosphating treatments, it is, of course, most desirable that the coloration of the surface be substantially identical to that produced by natural weathering of the zinc surface. Accordingly, for purposes of illustration, phosphating compositions producing this result will be considered.

A particularly useful phosphating solution would be within the purview of the following formulation:

0.5 gm NiCl 0.25 gm NaF 1.0 L H O pH adjusted to 3.0 with NaOH The grained zinc metal is exposed to the phosphating composition preferably by immersion but operably by spraying, flowing, etc., to produce the phosphate coating. Of course, it is necessary that the grained metal be treated without prolonged exposure to the atmosphere in that the phosphating coating, though essentially identical in appearance, is chemically dissimilar from the natural weathering of zinc. Thus, natural weathering may prove detrimental to the practice of the instant invention.

The time of exposure of the zinc to the phosphating solution is preferably between about 15 seconds and 2 minutes, and most desirably, about 30-40 seconds. The phosphating solution is preferably maintained at a temperature of about 140 F., plus or minus 10 F., though, as with all of the operating parameters, the exact conditions are somewhat dependent upon the phosphating solution employed. The two critical aspects of the phosphating process are the mechanical graining and, when it is desirable to substantially duplicate the appearance of natural weathering, the use of a nickel ion.

The method and results of the instant invention will be more readily understood upon consideration of the following examples:

EXAMPLE 1 Samples of zinc roofing material were cleaned by mechanical graining with stiff brushes and a pumice slurry and then treated by immersion in the following solution for one (1) minute at 140 F.

10 cc Phosphoric Acid 20 gm Zinc Nitrate 5 gm Zinc Chloride l L Water The coating produced was a very light gray but quite uniform in color and displayed corrosion resistance similar to naturally weathered zinc.

EXAMPLE 2 Samples of zinc roofing alloy were cleaned by mechanical graining and treated by immersion in the following solution for one (1) minute at 140 F.

5 cc Phosphoric Acid 1 gm Zinc Nitrate 0.25 gm Sodium Fluoride 0.5 gm Nickel Chloride l L Water The coating produced was very close in color to that of naturally weathered zinc and displayed corrosion resistance similar to naturally weathered zinc.

EXAMPLE 3 A series of samples were cleaned by the following processes to determine the best technique.

Trichlorethylene degrease Hydrochloric Acid Pichle Sulfuric Acid Pichle Oakite 332 (Zinc Cleaner) Detergent Sprex (Zinc Cleaner) Terj Zt (Zinc Cleaner) lHA-lA (Zinc Cleaner) Mechanical graining pumice, quartz, TSP, water On all of the samples, with the exception of those cleaned mechanically, the uniformity of coating color produced was unsatisfactory.

EXAMPLE 4 A test roof of zinc panels pre-weathered using:

1. Mechanical graining 2. Rinsing 3. A phosphating bath similar to that of Example 2 4. Rinsing 5. Drying has been fabricated using a standing seam design. The interior of the building has been maintained at F. and 70 percent relative humidity minimums for four (4) months. No changes have been observed in the character or appearance of the zinc panels.

EXAMPLE 5 Samples of zinc roofing alloy were cleaned by mechanical graining and treated by immersion in the following non-phosphating solution for one (1) minute at F.

60 gm Nickel Ammonium Sulfate 60 gm Ammonium Chloride l L Water The processing produced a purple color on the zinc samples. The coating showed signs of deterioration after 24 hours exposure to the atmosphere.

EXAMPLE 6 Samples of zinc roofing alloy were cleaned by mechanical graining and treated with the same solution as in Example 1 but at 140 F.

The processing produced a brown color on the zinc samples. The coating showed signs of deterioration after 24 hours exposure to the atmosphere.

EXAMPLE 7 Samples of zinc roofing alloy were cleaned by mechanical graining and treated by immersion in the following solution for one (1) minute at 120 F.

60 gm Colbalt Ammonium Sulfate 60 gm Ammonium Chloride l L Water The color produced by this process was a steel bluegray color. The coating exhibited signs of corrosion after 12 hours of atmospheric exposure- From the above examples, it can be seen that nonphosphating color producing coating solutions (Examples 5, 6 and 7) are not satisfactory. Further, ordinary zinc cleaning means relying onsolvent or chemical mechanisms (Example 3) do not produce the desirable results obtainable only with a physical abrading of the zinc surface, such as a mechanical graining. The basic results of the instant invention are obtainable with a simple phosphating solution (Example 1), while a coloration similar to that resulting from natural weathering requires the inclusion of a nickel ion (Example 2). The exact composition of the phosphating solution is not critical and may be varied. Conventional accelerators, such as nitrates and fluorides, are usually utilized. The coloration of the instant invention is essentially permanent (Example 4). It is not known if the phosphate coating, upon aging, reverts to the natural carbonate coating. Whatever the mechanism, the instant invention provides the asserted desirable results.

The specific mechanical graining and phosphating steps are well-known to those skilled in the art. It will be recognized that it is desirable to rinse the metal after both steps to remove residue. Also, it may be desirable to subject the final, dried metal to an oil wash, in such as mineral oil, to remove a slight powdery haze which sometimes appears. This is, however, optional.

It will be apparent from the above description that various modifications in the specific materials, articles and methods described in detail may be made within the scope of the invention. Therefore, the invention is not intended to be limited to the particular designs, procedures and formulations, except as may be required by the following claims.

What is claimed is:

1. A method for producing an even, corrosion resistant coating on a metal surface primarily of zinc comprising mechanically graining at least a portion of the metal surface and exposing the mechanically grained metal surface to zinc ion-containing phosphating solutions whereby a smooth, even corrosion resistant phosphate coating is formed on the surface.

2. A method as set forth in claim 1 wherein the phosphating solution incudes a nickel ion and the resulting phosphate coating is of a gray color similar to that resulting from the natural weathering of zinc.

3. A method as set forth in claim 1 wherein the mechanical graining is accomplished by abrading the metal surface utilizing a pumice slurry.

4. A method for treating a zinc metal surface comprising me'chanically graining the surface with stiff brushes and a pumice slurry and applying an aqueous zinc ion-containing phosphating solution comprising phosphoric acid and nitrate and chloride accelerators.

5. A method as set forth in claim 4 wherein the phosphating solution incudes a nickel ion.

6. A method of forming a smooth, even coating on a zinc surface comprising mechanically graining the surface and applying a phosphating solution comprising 5 cc phosphoric acid, 0.5 grams NiCl 1 gram Zn(NO 0.25 grams NaF and 1 liter of water, the solution being adjusted to a pH of 3 to 4, whereby the resulting corrosion resistant coating is of a gray color closely resembling that produced by the natural weathering of zinc.

7. An article comprising a metal surface comprising zinc, said surface being mechanically grained, and an even, corrosion resistant phosphate coating on the mechanically grained surface.

8. An article as set forth in claim 7 wherein the phosphate coating includes nickel and is of a gray color closely approximating that produced by the natural weathering of zinc.

9. An article comprising a metal sheet primarily of zinc, at least a portion of said sheet being grained and having thereon an even, gray phosphate coating.

10. An article as set forth in claim 9 wherein the phosphate coating comprises nickel whereby the gray coloration is identical to that occuring upon the natural weathering of zinc. 

2. A method as set forth in claim 1 wherein the phosphating solution incudes a nickel ion and the resulting phosphate coating is of a gray color similar to that resulting from the natural weathering of zinc.
 3. A method as set forth in claim 1 wherein the mechanical graining is accomplished by abrading the metal surface utilizing a pumice slurry.
 4. A method for treating a zinc metal surface comprising mechanically graining the surface with stiff brushes and a pumice slurry and applying an aqueous zinc ion-containing phosphating solution comprising phosphoric acid and nitrate and chloride accelerators.
 5. A method as set forth in claim 4 wherein the phosphating solution incudes a nickel ion.
 6. A method of forming a smooth, even coating on a zinc surface comprising mechanically graining the surface and applying a phosphating solution comprising 5 cc phosphoric acid, 0.5 grams NiCl2, 1 gram Zn(NO3)2, 0.25 grams NaF and 1 liter of water, the solution being adjusted to a pH of 3 to 4, whereby the resulting corrosion resistant coating is of a gray color closely resembling that produced by the natural weathering of zinc.
 7. An article comprising a metal surface comprising zinc, said surface being mechanically grained, and an even, corrosion resistant phosphate coating on the mechanically grained surface.
 8. An article as set forth in claim 7 wherein the phosphate coating includes nickel and is of a gray color closely approximating that produced by the natural weathering of zinc.
 9. An article comprising a metal sheet primarily of zinc, at least a portion of said sheet being grained and having thereon an even, gray phosphate coating.
 10. An article as set forth in claim 9 wherein the phosphate coating comprises nickel whereby the gray coloration is identical to that occuring upon the natural weathering of zinc. 